A system for locating a horizontal bore below a ground surface includes a transmitting source that radiates from the bore a dipole magnetic field aligned with the bore. A receiver includes a first and a second coil, the axis of each coil being orthogonal to the axis of the other. A measurement device...http://www.google.ca/patents/US6717410?utm_source=gb-gplus-sharePatent US6717410 - Bore location system
